WA Seasonal Rainfall Outlook: probabilities for April to June 2012,
issued 21st March 2012

No strong rainfall signal for WA

The Western Australian outlook for April to June 2012 shows the following:

roughly even chances of a wetter or drier season

This outlook is the result of warmer than normal waters over the Indian Ocean.

The chances of receiving above median rainfall
for April to June are between
40 and 55% across WA, indicating that the chance of an above or below average
three months is roughly equal.

An expanded set of seasonal rainfall outlook maps and tables, including the probabilities of seasonal rainfall
exceeding given totals (e.g. chance of receiving at least 200 mm), is available on the
"Water and the Land" (WATL) part of the Bureau's website.

Outlook confidence is
related to how consistently the Pacific
and Indian Oceans affect Australian rainfall.
During the April to June period, history shows the effect to be only weakly consistent across much of
WA, apart from some areas of central and southeastern WA where the effect is moderately consistent.
(see background information).
Users should exercise caution when using this outlook in areas of low skill.

The 2011-12 La Niña event is nearing its end, with most indicators approaching or at neutral
values. Climate models surveyed by the Bureau of Meteorology suggest that the tropical Pacific
Ocean will continue to warm over the coming months, with a neutral ENSO state expected to persist
at least through the second half of autumn.
For routine updates and comprehensive discussion on any developments regarding
El Niño and La Niña, please see the ENSO Wrap-Up.

Click on the map above for a larger version of the map. Use the reload/refresh button to ensure the latest forecast map is displayed.

More information on this outlook is available from
8.30am to 4.30pm (WST) Monday to Friday by
contacting the Climate Services Centre in
the Bureau's Perth Office: (08) 9263 2222.

Background Information

The Bureau's seasonal outlooks are general statements about the probability
or risk of wetter or drier than average weather over a three-month period.
The outlooks are based on the statistics of chance (the odds)
taken from Australian rainfall/temperatures
and sea surface temperature records for the tropical Pacific and Indian Oceans.
They are not, however, categorical predictions about future rainfall,
and they are not about rainfall within individual months
of the three-month outlook period.
The temperature outlooks are for the average maximum and minimum temperatures
for the entire three-month outlook period.
Information about whether individual days or weeks may be
unusually hot or cold, is unavailable.

This outlook is a summary.
More detail is available from the contact people.

Probability outlooks should not be used as if they were categorical forecasts.
These outlooks should be used as a tool in risk management
and decision making.
The benefits accrue from long-term use, say over 10 years.
At any given time, the probabilities may seem inaccurate,
but taken over several years, the advantages of taking account
of the risks should outweigh the disadvantages.
For more information on the use of probabilities,
farmers could contact their local departments of agriculture or primary industry.

Model Consistency and Outlook Confidence:
Strong consistency means that tests of the model on historical
data show a high correlation between the most likely
outlook category (above/below median)
and the verifying observation (above/below median). In this
situation relatively high confidence can be placed in the
outlook probabilities.
Low consistency means the historical relationship, and
therefore outlook confidence, is weak.
In the places and seasons where the outlooks are most skilful,
the category of the eventual outcome (above or below median)
is consistent with the category favoured in the outlook
about 75% of the time.
In the least skilful areas,
the outlooks perform no better than random chance or guessing.
The rainfall outlooks perform best in eastern and
northern Australia between July and January,
but are less useful in autumn and in the west of the continent.
The skill at predicting seasonal maximum temperature peaks in
early winter and drops off marginally during the second half of the year.
The lowest point in skill occurs in early autumn.
The skill at predicting seasonal minimum temperature peaks in
late autumn and again in mid-spring.
There are also two distinct periods when the skill is lowest
- namely late summer and mid-winter.
However, it must always be remembered that the outlooks
are statements of chance or risk.
For example, if you were told there was a 50:50 chance
of a horse winning a race but it ran second,
the original assessment of a 50:50 chance could still have been correct.

The Southern Oscillation Index (SOI) is calculated using the
barometric pressure difference between Tahiti and Darwin.
The SOI is one indicator of the stage of El Niño or La Niña
events in the tropical Pacific Ocean.
It is best considered in conjunction with sea-surface temperatures,
which form the basis of the outlooks.
A moderate to strongly negative SOI (persistently below −10) is
usually characteristic of El Niño, which is often associated with below
average rainfall over eastern Australia, and a weaker than normal monsoon
in the north. A moderate to strongly positive SOI (persistently above +10)
is usually characteristic of La Niña, which is often associated with
above average rainfall over parts of tropical and eastern Australia,
and an earlier than normal start to the northern monsoon season.
The Australian impacts of past El Niño events since 1900 are summarized
on the Bureau's web site
(El Niño - Detailed
Australian Analysis), and past La Niña events (La Niña -
Detailed Australian Analysis)